It is well-known that glass in its pristine condition is a very strong materia, but that scratches and abrasion on the glass surface will considerably decrease its strength. Consequently, glass articles, for example, containers such as jars, bottles, tumblers, and the like which have an impermeable non-porous glass surface, have maximum strength immediately after formation; however, this strength diminishes when the glass article surface contacts other surfaces as may occur during the inspection, handling, packaging, shipping, and consumer use of the article.
To overcome this problem, there has been a great deal of research in the glass industry towards development of thin, tenaciously adhering, lubricious, damage-preventive coatings which preserve the glass strength and allow the glass article to be handled and used by the consumer.
In the glass container industry, such damage-preventive coatings have been primarily of two types. In one type, the container is coated with a thin organic coating at the "cold end" of the annealing lehr where the temperature is in the range of 200.degree.-400.degree. F. These coatings have been water soluble polyoxyethylene stearate as in U.S. Pat. No. 2,813,045; polyethylene as in U.S. Pat. No. 2,995,533; and U.S. Pat. No. 2,965,596; or other organic materials as in U.S. Pat. Nos. 3,445,275; 3,487,035; 3,357,853 and 3,296,174.
In the second type of coating, the glass container is first coated with metal oxides such as the oxides of tin, titanium, vanadium, or zirconium at the hot end of the annealing lehr where the temperature is in the range of 1000.degree. to 1100.degree. F. and then overcoated with a protective organic coating at the cold end of the lehr. Such dual coatings are illustrated in U.S. Pat. Nos. 3,323,889; 3,425,859; 3,598,632; 3,554,787; 3,498,825; 3,418,154; 3,420,693; 3,445,269; 3,407,085; 3,414,429; and 3,352,707. The above types of coatings are "production line" coatings because their application is accomplished as part of the forming and annealing sequence. The disclosures of these patents are incorporated herein by reference.
The success of some of these and other types of coatings, particularly those "cold end" coatings of polyethylene or polyoxyethylene stearate (with or without the "hot end" coating of TiO.sub.2 or SnO.sub.2) in preserving the strength of the glass containers during inspection, handling, filling, and use has permitted glass researchers to focus on the problems caused by mishandling and accidental misuse of glass containers by the consumer.
Glass containers are sometimes fractured by dropping or other accidental misuse. This problem is particularly acute when the glass container has pressurized contents such as in the case of beer or carbonated soft drinks.
In accordance with this invention, there is provided a coating for glass containers which is capable of retaining broken glass fragments upon fracture of the glass container so as to reduce the incidence of accidental injury.
Attention has been directed to this problem in the recent past. For instance, German patent disclosure No. 2,026,909, published Dec. 10, 1970, discloses coating a glass container with a loosely or firmly adhering plastic material designed to form a "bag" which retains glass fragments when the container is broken. The film is formed by fusing powdered polyethylene to the glass bottle. German patent document No. 2,149,219, published May 25, 1972, discloses coating glass containers with a film coating of a hydrolyzed ethylene-vinyl acetate copolymer. U.S. Pat. No. 3,178,049 discloses a light, composite glass container having a wall thickness of about 0.15 to 0.70 millimeters surrounded on the outside by an envelope of a thermoplastic material having a wall thickness at least equal to the glass. U.S. Pat. No. 3,415,673 discloses glass containers which are made resistant to breakage by coating the exterior surface with a thin, highly adhesive layer of plastic consisting essentially of ethylene and acrylic copolymers. A primer is used to tenaciously adhere the copolymer to the surface of the glass article.
Studies of the effect of organic coatings on the fragmentation of glass substrates have demonstrated that soft, flexible, "rubbery" polymeric coatings are extremely effective in retaining fragments. However, such coatings are not ordinarily suitable for use on glass containers, owing to their poor abrasion and mar resistance, low lubricity, and surface tack, which preclude the handling of such coated containers on automatic filling lines. The present invention improves the physical properties and surface characteristics of such coating materials through chemical crosslinking.